The invention relates to optical isolators and optical circulators, important nonreciprocal devices in optical communications.
An optical isolator transmits light in one direction and blocks it in the other direction. It is used, for example, to protect active devices such as lasers from back-reflected light, which makes them unstable.
An optical circulator has multiple ports, e.g. port#1 to port#3, where the input from port#1 goes to port#2, the input from port#2 goes to port#3 and the input from port#3 goes to port#1. Many applications exist for circulators. For example, dispersion compensators and optical add/drop multiplexers (OADMs), both important devices in dense wavelength division multiplexing (DWDM) systems, can be realized using circulators and Bragg gratings.
Both isolators and circulators are called nonreciprocal devices because the backward propagating optical signal does not follow the same path as the forward propagating optical signal. Such devices can be realized only via magneto-optic effects such as Faraday rotation.
Most of the currently available polarization independent optical isolators and circulators are based on birefringent prisms or wedges, 45° Faraday rotators, and half-wave plates to spatially split, rotate the two orthogonal polarizations, and recombine the two split optical signals such as demonstrated in Shirasaki et al., “Compact polarization-independent optical circulator”, Appl. Opt. 20 no. 15, pp. 2683-87 (1981), as shown in FIG. 1. Light first goes through the lens 100 and becomes collimated. The two orthogonal polarizations from input #1 will be split into two separate paths by a first polarization beam splitter 102. For each polarization path, light goes through a 45° Faraday rotator 104, a half wave plate 106, a second polarization beam splitter 108, and a second lens 110. The Faraday rotator 104 always rotates the polarization axis of the incident light by 45° independent of the propagation direction, whereas the half-wave plate 106 rotates the polarization axis either by +45° or by −45° depending on its propagation direction. In this way, forward propagating light experiences a total polarization rotation of 0° between the beam splitters, and goes into port#2. The backward propagating light experiences 90° polarization rotation, i.e. it gets converted from the original polarization into its orthogonal counterpart, and therefore goes into port#3 instead of port#1.
This design requires elements such as birefringent beam splitters and lenses that require precise material preparation, positioning, and alignment, making the device expensive. Although there have been previous efforts to simplify the design of the polarization beam splitters, such as U.S. Pat. No. 4,464,022 issued to Emkey, most of the currently available devices still rely on bulky single-crystalline elements, such as prisms and lenses.
Recently, technologies have been developed for producing such optical devices in planar optical circuits. Advantages of such structures include easy mass production, reduced overall size and low-loss integration with other planar components, which leads to a significant advantage in cost and size compared to the individually packaged discrete components.
One isolator/circulator design is based on nonreciprocal mode conversion with a Faraday rotator and half-wave plates originally proposed by Shintaku et al. and U.S. Pat. No. 6,075,596 issued to Pan et al., as shown in FIG. 2. Their designs are based on a Mach-Zehnder interferometer where the polarization is rotated with the use of a first coupler 200, a Faraday rotator 202 across the two arms, half-wave plates 204 and 206 each in one arm, and a second coupler 208. By having the two wave plates on opposite sides of the Faraday rotator and at an angle of 45° between their slow axes, constructive and destructive interference in forward and backward propagation direction is achieved, respectively.
This design has an advantage over the previous one because it avoids the costly polarization beam splitters. However, their designs cannot be easily integrated into planar optical circuits because the currently available 45° Faraday rotators have a thickness of ˜0.5 mm or more, making costly lenses necessary in order to avoid large diffraction losses when coupling back into the waveguide. A lens assembly also requires high positioning accuracy and high mechanical stability. U.S. Pat. No. 5,905,823 issued to Shintaku et al. describes an optical circulator in a planar waveguide form with a waveguide Faraday rotator. However, a waveguide Faraday rotator requires tight birefringence control, and its fabrication is therefore extremely difficult. This problem is evident from the fact that there is no commercially available waveguide Faraday rotator.
The prior art also includes an isolator/circulator based on an interferometer with nonreciprocal phase shift via transverse magneto-optical effect using a 90° TM nonreciprocal phase shifter 300 and a 90° reciprocal phase shifter 302, as shown in FIG. 3. However, this device functions only for TM light and is therefore not suitable for practical applications.